The present invention finds its application in the grinding (resurfacing, to provide a smooth surface) of rubber belts which are used to compact and attenuate paper webs or other web material such as fabric. As known in the art, such compaction can in an appropriate paper web, increase tear, tensile energy absorption and burst strengths, and can effect shrinkage control in a fabric web.
Rubber belts used for this purpose are in the order of 1-2 inches in thickness; may exceed 80 inches in width, and when used in the paper industry have a Shore A durometer of 45-60. Compaction of the web (paper or fabric) occurs when the belt is stretched as it passes around a nip bar immediately prior to entering a compaction zone formed between the belt (carrying the web in frictional engagement therewith) and a steel roller against which the belt and paper slip. The web is foreshortened when the belt contracts in the compaction zone.
The foregoing takes place at production speeds which may, for example, be 1600 to 2500 feet/minute and because steam facilitates web compaction in the compaction zone, compaction takes place at fairly high temperatures. In such environment, constant flexing of the surface of the belt and rubbing of the belt against the steel roller in the compaction zone cause wrinkling and uneven wearing of the belt surface. This adversely affects the compaction process and reduces belt life.
Therefore, provision must be made periodically to resurface the paper contacting (outside) surface of the rubber belt since after a number of operating hours this surface will develop wrinkles which will transfer to the paper surface and the edges not covered by the paper will become hard and glazed, inviting surface cracks in the belt. These surface defects must be removed by grinding to restore a smooth working surface for proper operation of the compacting unit and to prolong the belt life.
A traversing belt type grinder (for example, grinder belt width =4 inches) has proved most satisfactory for machines over 80 inches in width (2032 mm) while a full width roll type grinder may be used, with proper design, on narrower machines. In either case, grinder belt grit of 60 to 120 is preferred for most grinding while a 50 grit may be used for removing a large amount of rubber. It is fairly common for rubber belts used to compact paper to be 200 inches in width. Current grinding practice is to position the grinder belt to initially grind the belt surface during one full rotation of the belt; and then to laterally move the grinder 2 inches (where the grinder is 4 inches wide) so that the grinder will overlay the previously ground surface by 2 inches. Therefore, if the rubber belt whose surface is being ground has a width of 200 inches, one hundred revolutions of the belt will be required to grind the entire surface.
Provision should be made for grinding to take place at the operator's option both during production at full production speed and with the unit operating at crawl drive speed of 80 fpm (24.38 m/min). When grinding is done during extensible paper production at full production speed, belt tension may be set for example at 40 lb/in (70 newtons/centimeter) or tension may depend upon the system. The belt will be hot when grinding during production so the grinder should be set for a very light cut during each traverse to prevent rubber dust from bailing up and becoming sticky. The average depth of cut should be no more than 0.05 mm during each traverse, just enough to cut the belt surface. Two or three traverses is usually all that is necessary to clean and refinish the belt surface with fresh, "live", rubber. During production grinding should be done at least once per production day to maintain a resilient, "live" surface. Currently, when heavier grinding has been required, it has been done at crawl speed which, for a 200 inch belt, takes about 3-4 hours. If this results in lost production time, the economic loss can be considerable since such loss can be 30 tons per hour at $1000 per ton for compacted paper production.
It has been found that present grinding systems do not properly lend themselves to operation during production use of the compacting (extensible) units. Operators are forced to work from traversing platforms which carry the grinding apparatus and which are subject to high heat. This often has necessitated such operators to leave the traversing platforms periodically with unsatisfactory and even disastrous results. Current resurfacing techniques are deficient as well since belt surface areas are normally removed under some form of imprecise visual control. Furthermore, present grinding apparatus has been known to cause hazardous explosive conditions due to improper control of particulates created by the grinding process. It is the purpose of the present invention to address these problems and to provide solutions therefor.